This correspondence between the internal / outer diameter and the nominal tube or pipe size is not exact. The pipe internal diameter is not exactly equal to nominal pipe size but deviates from the nominal diameter, depending on the schedule.

Similarly, the tube outer diameter is not exactly equal to the tube size, depending on tube wall thickness. Pipe wall thickness is defined by the 'pipe schedule' and tube wall thickness is defined by the 'Birmingham wire gauge' for tubes.

Other than the method and critical dimension used for defining nominal sizes, there is no difference between pipes and tubes. Pipes and tubes can be made from the same range of materials and can handle the same services.

The difference between pipes and tubes only comes into picture for defining the standard dimensions.